Internet DRAFT - draft-ietf-idr-add-paths-guidelines
draft-ietf-idr-add-paths-guidelines
IDR Working Group J. Uttaro
Internet-Draft AT&T
Intended status: Informational
Expires: Oct 25, 2016 P. Francois
IMDEA Networks
K. Patel
Cisco Systems
J. Haas
Juniper Networks
A. Simpson
R. Fragassi
Nokia
Apr 25, 2016
Best Practices for Advertisement of Multiple Paths in IBGP
draft-ietf-idr-add-paths-guidelines-08.txt
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html
This Internet-Draft will expire on October 25, 2016.
Uttaro, et al Expires Oct 25, 2016 [Page 1]
Internet-Draft draft-ietf-idr-add-paths-guidelines-08 Apr 2016
Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Abstract
Add-Paths is a BGP enhancement that allows a BGP router to advertise
multiple distinct paths for the same prefix/NLRI. This provides a
number of potential benefits, including reduced routing churn, faster
convergence and better loadsharing.
This document provides recommendations to implementers of Add-Paths
so that network operators have the tools needed to address their
specific applications and to manage the scalability impact of Add-
Paths. A router implementing Add-Paths may learn many paths for a
prefix and must decide which of these to advertise to peers. This
document analyses different algorithms for making this selection and
provides recommendations based on the target application.
Table of Contents
1. Introduction...................................................4
2. Terminology....................................................4
3. Add-Paths Applications.........................................5
3.1. Fast Connectivity Restoration.............................5
3.2. Load Balancing............................................7
3.3. Churn Reduction...........................................7
3.4. Suppression of MED-Related Persistent Route Oscillation...8
4. Implementation Guidelines......................................8
4.1. Capability Advertisement..................................8
4.2. Receiving Multiple Paths..................................9
4.3. Advertising Multiple Paths...............................10
4.3.1. Path Selection Modes................................11
4.3.1.1. Advertise N Paths..............................11
4.3.1.2. Advertise All Paths............................12
4.3.1.3. Advertise All AS-Wide Best Paths...............13
Uttaro, et al. Expires Oct 25, 2016 [Page 2]
Internet-Draft draft-ietf-idr-add-paths-guidelines-08 Apr 2016
4.3.1.4. Advertise ALL AS-Wide Best and Next-Best Paths
(Double AS Wide)........................................14
4.3.1.5. Advertise Used Multipaths......................14
4.3.2. Derived Modes from Bounding the Number of Advertised
Paths......................................................15
4.3.3. Derived Modes from Adding N More Paths..............15
5. Deployment Considerations.....................................16
5.1. Introducing Add-Paths into an Existing Network...........16
5.2. Scalability Considerations...............................18
5.3. Routing Consistency Considerations.......................18
5.4. Consistency between Advertised Paths and Forwarding Paths19
5.5. Interactions with Route Filtering........................20
5.6. Routing Churn............................................20
6. Security Considerations.......................................21
7. Acknowledgments...............................................21
8. Contributors..................................................21
9. IANA Considerations...........................................21
10. References...................................................21
10.1. Normative References....................................21
10.2. Informative References..................................22
Appendix A. Other Path Selection Modes...........................23
A.1. Advertise Neighbor-AS Group Best Path....................23
A.2. Best LocPref/Second LocPref..............................23
A.3. Advertise Paths at decisive step -1......................24
Uttaro, et al. Expires Oct 25, 2016 [Page 3]
Internet-Draft draft-ietf-idr-add-paths-guidelines-08 Apr 2016
1. Introduction
The BGP Add-Paths capability enhances current BGP implementations by
allowing a BGP router to exchange with its BGP peers more than one
path for the same destination/NLRI. The base BGP standard [RFC4271]
does not provide for such a capability. If a BGP router learns
multiple paths for the same NLRI (from multiple peers), it selects
only one as its best path and advertises the best path to its peers.
The primary goal of Add-Paths is to increase the visibility of paths
within an IBGP system. This has the effect of improving robustness
in case of failure, reducing the number of BGP messages exchanged
during such an event, and offering the potential for faster re-
convergence. Through careful selection of the paths to be advertised,
Add-Paths can also prevent routing oscillations.
The purpose of this document is to provide the necessary
recommendations to the implementers of Add-Paths so that network
operators have the tools needed to address their specific
applications and to manage the scalability impact of Add-Paths while
maintaining routing consistency. A router implementing Add-Paths may
learn many paths for a prefix and must decide which of these to
advertise to peers. This document analyses different algorithms for
making this selection and provides recommendations based on the
target application.
2. Terminology
In this document the following terms are used:
Add-Paths peer: refers a peer with which the local system has agreed
to receive and/or send NLRI with path identifiers
Primary path: A path toward a prefix that is considered a best path
by the BGP decision process [RFC4271] and actively used for
forwarding traffic to that prefix. A router may have multiple primary
paths for a prefix if it implements multipath.
Diverse path: A BGP path associated with a different BGP next-hop and
BGP router than some other set of paths. The BGP router associated
with a path is inferred from the ORIGINATOR_ID attribute or, if there
is none, the BGP Identifier of the peer that advertised the path.
Backup path: A diverse path with respect to the primary paths toward
a prefix. The backup path can be used to forward traffic to the
destination if the primary paths fail.
Uttaro, et al. Expires Oct 25, 2016 [Page 4]
Internet-Draft draft-ietf-idr-add-paths-guidelines-08 Apr 2016
Optimal backup path: The backup path that will be selected as the new
best path for a prefix when all primary paths are removed/withdrawn.
AS-Wide preferred paths: All paths that are considered as best when
applying rules of the BGP decision process up to the IGP tie-break.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
3. Add-Paths Applications
[draft-pmohapat] presents the applications that would benefit from
multiple paths advertisement in IBGP. They are summarized in the
following subsections.
3.1. Fast Connectivity Restoration
With the dissemination of backup paths, fast connectivity restoration
and convergence can be achieved. If a router has a backup path, it
can directly select that path as best upon failure of the primary
path. This minimizes packet loss in the dataplane. Sending multiple
paths in IBGP allows routers to receive backup paths when path
visibility is not sufficient with classical BGP. This is especially
useful when Route Reflection is used.
Consider a network such as the one depicted in Figure 1 and suppose
that none of the routers support Add-Paths. AS1 receives from AS3 two
paths (A and B) to a particular destination in AS3. Suppose path A is
preferred over path B due to path A having a lower MED (multi-exit
discriminator).
AS1 uses a route reflector RR1 to reduce the scale of its IBGP mesh.
If the routers in AS1 are not configured to advertise the best
external path [BEST-EXT] then RR1 knows about only path A during
steady state because router B suppresses/withdraws its advertisement
of path (B) to RR1. If the routers in AS1 do support best-external
[BEST-EXT] then RR1 may have both paths in its Adj-RIB-IN, but
regardless, RR1 can only advertise its best path A to its peers,
including router D.
Uttaro, et al. Expires Oct 25, 2016 [Page 5]
Internet-Draft draft-ietf-idr-add-paths-guidelines-08 Apr 2016
======== =====================
= +---+ +---+ +---+
= |RTR|________|RTR| |RTR|
= | E | | A | | C |
= +---+Path A->+---+ AS1 +---+
= = = \ / =
= = = \ / =
= = = \ / =
= = = \ / =
= AS3 = = +---+ =
= = = |RR | =
= = = | 1 | =
= = = +---+ =
= = = / \ =
= = = / \ =
= = = / \ =
= = = / \ =
= +---+Path B->+---+ +---+
= |RTR| ______|RTR| |RTR|
= | F | | B | | D |
= +---+ +---+ +---+
======== =====================
Figure 1: Example Topology
Under these circumstances consider the steps required to restore
traffic from router D to the destination in AS3 when the link between
Router A and Router E fails. (Assume that router A set next-hop to
self when advertising path A and that router B is not configured for
best-external [BEST-EXT]).
1. Router A sends a BGP UPDATE message to RR1 withdrawing its
advertisement of path (A).
2. RR1 receives the withdrawal, and propagates it to its other client
peers, routers B, C and D.
3. When router B receives the withdrawal of path (A) it reruns its
decision process and selects path (B) as its new best path. Router
B advertises path (B) to RR1.
4. RR1 reruns its decision process and selects path (B) as its new
best path. RR1 advertises path (B) to client peers A, C and D.
Uttaro, et al. Expires Oct 25, 2016 [Page 6]
Internet-Draft draft-ietf-idr-add-paths-guidelines-08 Apr 2016
5. Router D reruns its decisions process, determines path (B) to be
the best path, and updates its forwarding table. After this step
traffic from router D to the destination in AS3 is restored (the
traffic path has changed to go through router B and then router
F).
With the use of Add-Paths, the convergence time for the above path
failure example can be reduced considerably. The main reason for the
improvement is that Add-Paths allows router D to be aware of more
than one path to the destination in AS3 prior to the failure of the
best path (A). In steady-state (with no failures) router B decides,
as before, that path (A) is its best path but because of its Add-
Paths (or best-external [BEST-EXT]) configuration it also advertises
path (B) to RR1. Using Add-Paths RR1 can advertise both learned paths
to its IBGP peers, including router D. Now consider again the
scenario where the link between Router A and Router E fails. In this
case, with Add-Paths, fewer steps are required to achieve re-
convergence:
1. Router A sends a BGP UPDATE message to RR1 withdrawing its
advertisement of path (A).
2. RR1 receives the withdrawal, and propagates it to its other client
peers, routers B, C and D.
3. Router D receives the withdrawal, reruns the decision process and
updates the forwarding entry for the destination in AS3.
4. Traffic from router D to the destination in AS3 follows the
alternate path through router B-router F provided that the other
routers on this forwarding path have the same synchronized IP FIB
view or else encapsulation is used to avoid IP FIB lookups until
traffic reaches router F; refer to section 5.4 for more discussion
of this topic.
3.2. Load Balancing
Increased path diversity allows routers to install several paths in
their forwarding tables in order to load balance traffic across those
paths. The forwarding consistency considerations mentioned in section
5.4 also apply to this use case.
3.3. Churn Reduction
When Add-Paths is used in an AS, the availability of additional
backup paths means failures can be recovered locally with much less
path exploration in IBGP and less intermediate updates sent to EBGP
Uttaro, et al. Expires Oct 25, 2016 [Page 7]
Internet-Draft draft-ietf-idr-add-paths-guidelines-08 Apr 2016
peers. When the preferred backup path is the post-convergence path,
churn is minimized.
3.4. Suppression of MED-Related Persistent Route Oscillation
As described in [oscillation], Add-Paths is a valuable tool in
helping to stop persistent route oscillations caused by comparison of
paths based on MED in topologies where route reflectors or the
confederation structure hide some paths. With the appropriate path
selection algorithm Add-Paths stops these route oscillations because
the same set of paths are consistently advertised by the route
reflector or the confederation border router and the routers
receiving this set of paths make stable routing decisions about the
best path.
4. Implementation Guidelines
This section discusses recommendations for the implementation of Add-
Paths. The following topics are addressed:
. Considerations related to Add-Paths capability negotiation
. Receiving BGP routes from Add-Paths peers
. Advertising BGP routes to Add-Paths peers. This section
discusses various path selection algorithms, which are the
procedures available to an Add-Paths speaker for deciding which
set of paths to advertise to an Add-Paths peer for particular
prefixes.
4.1. Capability Advertisement
+---+ +---+
|RTR|___________|RTR|
| A | <-BGP-> | B |
+---+ +---+
Figure 2: BGP Peering Example
In Figure 2, in order for a router A to receive multiple paths per
NLRI from peer B, for a particular address family (AFI=x, SAFI=y),
the BGP capabilities advertisements during session setup MUST
indicate that peer B wants to send multiple paths for AFI=x, SAFI=y
and that router A is willing to receive multiple paths for AFI=x,
SAFI=y. Similarly, in order for router A to send multiple paths per
NLRI to peer B, for a particular address family (AFI=x, SAFI=y), the
Uttaro, et al. Expires Oct 25, 2016 [Page 8]
Internet-Draft draft-ietf-idr-add-paths-guidelines-08 Apr 2016
BGP capabilities advertisements MUST indicate that router A wants to
send multiple paths for AFI=x, SAFI=y and peer B is willing to
receive multiple paths for AFI=x, SAFI=y. Refer to [Add-Paths] for
details of the Add-Paths capabilities advertisement.
The capabilities of the local router MUST be configurable per peer
and per address family, and SHOULD support the ability to configure
send-only operation or receive-only operation. The default mode of
operation is to both send and receive.
4.2. Receiving Multiple Paths
Currently, per standard BGP behavior, if a BGP router receives an
advertisement of an NLRI and path from a specific peer and that peer
subsequently advertises the same NLRI with different path information
(e.g. a different NEXT_HOP and/or different path attributes) the new
path effectively overwrites the existing path.
When Add-Paths has been negotiated with the peer, the newly
advertised path should be stored in the RIB-IN along with all of the
paths previously advertised (and not withdrawn) by the peer.
When an Add-Paths speaker has negotiated to receive multiple paths
for (AFIx, SAFIy) from a peer all advertisements and withdrawals of
NLRI within that address family from that peer MUST include a path
identifier, as described in [Add-Paths]. The path identifiers have no
significance to the receiving peer. If the combination of NLRI and
path identifier in an advertisement from a peer is unique (does not
match an existing route in the RIB-IN from that peer) then the route
is added to the RIB-IN. If the combination of NLRI and path
identifier in a received advertisement is the same as an existing
route in the RIB-IN from the peer then the new route replaces the
existing one. If the combination of NLRI and path identifier in a
received withdrawal matches an existing route in the RIB-IN from the
peer then that route shall be removed from the RIB-IN.
A BGP UPDATE message from an Add-Paths peer could advertise and
withdraw more than one NLRI belonging to one or more address
families. The receiving BGP router MUST NOT expect or require its
peer to send path identifiers with all routes belonging to all
address families. It also MUST NOT expect that all of the received
path identifiers in the UPDATE message are the same.
Uttaro, et al. Expires Oct 25, 2016 [Page 9]
Internet-Draft draft-ietf-idr-add-paths-guidelines-08 Apr 2016
4.3. Advertising Multiple Paths
[Add-Paths] specifies how to encode the advertisement of multiple
paths towards the same NLRI over an IBGP session, but provides no
details about which set of multiple paths should be advertised. In
this section, four path selection algorithms are described and
compared with each other. These 4 algorithms are considered to be the
most useful across the widest range of deployment scenarios. The list
of possible path selection algorithms is much larger and for the
interested reader Appendix A provides information about other path
selection modes that were considered in historical versions of this
document.
In comparing any two path selection algorithms the following factors
should be taken into account:
Control Plane Load: When a router receives multiples paths for a
prefix from an IBGP client it has to store more paths in its Adj-Rib-
Ins.
Control Plane Stress: Coping with multiple IBGP paths has two
implications on the computation that a router has to handle. First,
it has to compute the paths to send to its peers, i.e. more than the
best path. Second, it also has to handle the potential churn related
to the exchange of those multiple paths.
MED/IGP oscillations: BGP sometimes suffers from routing oscillations
when the physical topology differs from the logical topology, or when
the MED attribute is used. This is due to the limited path
visibility when a single path is advertised and Route Reflection is
used. Increasing the path visibility by advertising multiple paths
can help solve this issue.
Path optimality: When a single path is advertised, border routers do
not always receive the optimal path. As an example, Route Reflectors
typically send a single path chosen based on their own IGP tie-
breaking procedure (although modifications to this are proposed in
[BGP-ORR]). Increasing path visibility would also help routers to
learn the path that is best suited for them w.r.t. the IGP tie-
breaking.
Backup path optimality: Multiple paths advertisement gives routers
the opportunity to have a backup path. However, some backup paths
are better than others. Indeed, when a link failure occurs, if a
router already knows its post-convergence path, the BGP re-
convergence is straightforward and traffic is less impacted by the
transient use of non-best forwarding paths.
Uttaro, et al. Expires Oct 25, 2016 [Page 10]
Internet-Draft draft-ietf-idr-add-paths-guidelines-08 Apr 2016
Convergence time: Advertising multiple paths in IBGP has an impact on
the convergence time of the BGP system. More paths need to be
exchanged, but on the other hand, the routing information is
propagated faster. With an increased path visibility, there is less
path exploration during the convergence. Also, with the availability
of backup paths, convergence time in case of failure is also reduced.
Target application: Depending on the application type, the number of
paths to advertise for a prefix will vary. For example, for fast
connectivity restoration, it may be sufficient to advertise only 2
paths to a peer so that it will have the best path and the optimal
backup path. For load balancing purposes, it may be desirable to
advertise more paths, but inclusion of the optimal backup path in the
set may be less critical. For route oscillation elimination, it is
required to advertise all group-best paths for a prefix.
4.3.1. Path Selection Modes
The following subsections describe the 4 main path selection modes
considered in this draft. Each mode is considered either MANDATORY or
OPTIONAL. A MANDATORY mode MUST be supported by any implementation
that claims compliance with this document. An OPTIONAL made may be
supported by some but not all implementations.
The path selection mode and any parameters applicable to the mode
MUST be configurable per AFI/SAFI and per peer and SHOULD be
configurable per prefix. To illustrate the value of this flexibility,
consider a prefix P that belongs to an address family F requiring
path IDs to be included with every NLRI (e.g. due to the Add-Paths
capability negotiation with the peer). If P is one of a number of
prefixes that would not benefit from the advertisement of multiple
paths then it is perfectly valid to send only the best path.
4.3.1.1. Advertise N Paths
With the 'Advertise N Paths' mode (Add-N for short) a router
advertises up to N paths per prefix towards an Add-Paths peer. The
computational cost of this mode is the selection of the N paths.
There must be a ranking of the paths in order to ensure consistency
in the set of paths advertised to different Add-Paths peers. The
recommended way for a router to consistently select N paths is to run
its decision process N times and consider at each iteration only the
paths that meet all of the following criteria:
(a) not selected during a previous iteration
Uttaro, et al. Expires Oct 25, 2016 [Page 11]
Internet-Draft draft-ietf-idr-add-paths-guidelines-08 Apr 2016
(b)_diverse with respect to previously selected paths (see section
2 for the definition of a diverse path)
(c) not rejected by route filters or split horizon advertisement
rules
The memory cost of this path selection mode is bounded: a router
receives a maximum of N paths for each prefix from each peer. With N
equal to 2, all routers know at least two paths and can provide local
recovery in case of failure. If multipath routing is to be deployed
in the AS, N can be increased to provide more alternate paths to the
routers.
Path optimality and backup path optimality are not guaranteed, i.e.
it is possible that the optimal path of a router (w.r.t. IGP tie-
breaking) is not contained in the set of paths advertised by its
Route Reflector. However, as the number of paths that it receives is
higher than without Add-Paths, it is possible that the chosen nexthop
is closer to the router in terms of IGP cost than the nexthop that
would have been chosen without Add-Paths.
This solution helps to reduce routing oscillations, but not in all
cases. Indeed, path visibility is still constrained by the maximum
number of paths, and configurations with routing oscillations still
exist.
This path selection mode is MANDATORY. The default value of N MUST be
2. The value of N MUST be configurable and MAY be upper bounded by
an implementation.
The default value of 2 ensures the availability of a backup path (if
2 or more paths have been received) while maintaining minimum impact
to memory and churn. If Add-N with N equal to 2 is insufficient to
meet another objective (e.g. loadsharing or MED/IGP oscillation)
there is always a large enough value of N that can selected, if N is
configurable, to meet that objective.
4.3.1.2. Advertise All Paths
A simple rule for advertising multiple paths in IBGP is to advertise
to IBGP peers all received paths minus those blocked by export
filters or applicable split horizon rules. This solution is easy to
implement, but the counterpart is that all those paths need to be
stored by all routers that receive them, which can be quite
expensive. If a path to a prefix P is advertised to N border
routers, with a Full Mesh of IBGP sessions, all routers have N paths
Uttaro, et al. Expires Oct 25, 2016 [Page 12]
Internet-Draft draft-ietf-idr-add-paths-guidelines-08 Apr 2016
in their Adj-RIB-Ins. If Route Reflection is used and each client is
connected to 2 Route Reflectors, it may learn up to 2*N paths.
This solution gives a perfect path visibility to all routers, thus
limiting churn and losses of connectivity in case of failure. Indeed,
this allows routers to select their optimal primary path, and to
switch on their optimal backup path in case of failure.
However, as more paths are exchanged, the number of BGP messages
disseminated during the initial IBGP convergence can be high, and
convergence may be slower.
Routing oscillations are prevented with this rule, because a router
won't need to withdraw a previously advertised path when its best
path changes.
This path selection mode is OPTIONAL.
4.3.1.3. Advertise All AS-Wide Best Paths
Another choice is to consider the set of paths with the same AS-wide
preference [Basu-ibgp-osc], i.e. the paths that all routers would
select based on the rules of the decision process that are not
router-dependent (i.e. Local-preference, ASPath length and MED
rules). Thus, for a given router, those paths only differ by the IGP
cost to the nexthop or by the tie-breaking rules. The paths actually
advertised to a peer are the set of AS-wide best paths minus those
blocked by export filters or applicable split horizon rules.
The computational cost is reduced, as a router only has to send the
paths remaining before applying the IGP tie-breaking rule. However,
it is difficult to predict how many paths will be stored, as it
depends on the number of EBGP sessions on which this prefix is
advertised with the best AS-wide preference.
With this rule, the routing system is optimal: all routers can choose
their best path (or best paths if multipath is used) based on their
router-specific preferences, i.e. the IGP cost to the nexthop. Hot
potato routing is respected. Also, MED oscillations are prevented,
because the path visibility among the AS-wide preferred paths is
total.
The existence of a backup path is not guaranteed. If only one path
with the AS-wide best attributes exists, there is no backup path
disseminated. However, if such a path exists, it is optimal as it
has the same AS-wide preference as the primary
Uttaro, et al. Expires Oct 25, 2016 [Page 13]
Internet-Draft draft-ietf-idr-add-paths-guidelines-08 Apr 2016
This path selection mode is OPTIONAL.
4.3.1.4. Advertise ALL AS-Wide Best and Next-Best Paths (Double
AS Wide)
This variant of "Advertise All AS Wide Best Paths" trades-off the
number of paths being propagated within the iBGP system for post-
convergence alternate paths availability and routing stability. A BGP
speaker running this mode will select, as candidates for
advertisement, its AS Wide Best paths, plus all the AS Wide Best
paths obtained when removing the first ones from consideration. The
paths actually advertised to a peer are the double-AS_wide candidate
paths minus those blocked by export filters or applicable split
horizon rules.
Under this mode, a BGP speaker knows multiple AS-Wide best paths or
the AS-Wide best path and all the second AS-Wide best paths, so that
routing optimality and backup path availability are ensured. Note
that the post-convergence paths will be known by each BGP node in an
AS supporting this mode.
The computation complexity of this mode is relatively low as it
requires the router to run the usual BGP Decision Process up to and
including the MED rule. The set of paths remaining after that step
form the AS-Wide best paths. Next, a best path selection algorithm
is run up to and including the MED rule, based on the paths that are
not in the set of AS-Wide best paths.
The number of paths for a prefix p, known by a given router of the
AS, is the number of AS-Wide best and second AS-Wide best paths found
at the Borders of the AS.
MED Oscillations are avoided by this mode, both for the primary and
alternate paths being picked under this mode.
This path selection mode is OPTIONAL.
4.3.1.5. Advertise Used Multipaths
Many BGP implementations support BGP Multipath, allowing a BGP router
to use multiple BGP next-hops for forwarding towards a prefix/NLRI
when the corresponding paths are considered equally preferred. In
cases where the deployment of Add-Paths is mostly aimed at providing
multiple paths for load balancing with BGP Multipath, a natural
approach for a BGP speaker supporting Add-paths is to advertise the
paths that are selected by its BGP multipath selection algorithm.
Uttaro, et al. Expires Oct 25, 2016 [Page 14]
Internet-Draft draft-ietf-idr-add-paths-guidelines-08 Apr 2016
BGP Multipath selection algorithms can vary depending on the
implementation and configuration options. An Add-Paths mode based on
BGP multipath is considered practical because it lets the BGP path
propagation be aligned with the load balancing objectives expressed
by the operator configuring BGP multipath.
In some deployment scenarios, it is likely that such a mode leads to
the selection and advertisement of a large number of paths for some
NLRI, and hence should be controlled as per the mechanism described
in section 4.3.2. In case the number of multipaths exceeds the upper
bound on the number of advertised paths the ones that should be
advertised are those with the highest degree of preference by the BGP
decision process. This can be achieved if the advertising router has
strictly ordered all of its paths.
This path selection mode is OPTIONAL.
4.3.2. Derived Modes from Bounding the Number of Advertised Paths
For some of the modes discussed in section 4.3.1 the number of paths
selected by the algorithm (M) is not predictable in advance, and
depends on factors such as network topology. For such modes,
implementations MAY support the ability to limit the number of
advertised paths to some value N that is less than M.
It must be noted that the resulting derivative mode may no longer
meet the properties stated in section 4.3.1 (which assumes N=M). This
is particularly true for the MED oscillation avoidance property. The
use of such bounds thus needs to be considered carefully in
deployments where MED oscillation avoidance is a key goal of
deploying Add-path. If fast recovery is the main objective then it is
reasonable and sufficient to set N to 2. If the main goal is
improved load-balancing then limiting N to number of ECMP paths
supported by the forwarding planes of the receiving routers is also a
reasonable practice.
4.3.3. Derived Modes from Adding N More Paths
Some modes discussed in section 4.3.1 may result in only one or a few
selected paths, depending on network topology and/or router
configuration, and this small number of paths may not meet minimum
requirements for backup path or load balancing purposes. When using
such modes implementations MAY support the ability to add N more
paths to the set returned by the basic selection algorithm as
described in section 4.3.1. The N more paths should be the N next-
best paths, as determined by the BGP decision process.
Uttaro, et al. Expires Oct 25, 2016 [Page 15]
Internet-Draft draft-ietf-idr-add-paths-guidelines-08 Apr 2016
It must be noted that the resulting derivative mode may no longer
meet the properties stated in section 4.3.1 (which assumes N=0).
5. Deployment Considerations
This section proposes a potential strategy for introducing Add-Paths
into an existing network and discusses considerations related to
scalability, routing consistency and routing churn.
5.1. Introducing Add-Paths into an Existing Network
There are many possible ways that Add-Paths can be introduced into an
existing deployed network. It is not a practical goal for this
document to list all of these options and discuss the pros and cons
for each one. It is however valuable to consider an example migration
strategy that may be relatively common among layer 3 service
providers that currently use route reflectors for scaling. This
example migration strategy is attractive for several reasons:
1. It involves incremental steps that allow the impact of Add-
Paths to be carefully evaluated before proceeding to the next
step.
2. It recognizes the fact that many routers will require at least
a software upgrade to support Add-Paths, and it will not be
practical to upgrade all of these routers all at once.
3. It reduces convergence time (in stages) with a relatively
moderate increase in router memory and CPU demands.
The example migration strategy assumes a starting point of a deployed
network with one or more RR clusters. None of the routers in the
network support Add-Paths without an upgrade, but some do support
best-external. Two of the clusters in this network are shown in
Figure 3. In cluster 2, PE1, PE2, RRy and RRz are configured for
best-external. This makes RRy and RRz aware of all external paths
received by PEs in cluster 2 and ensures that RRy and RRz can
advertise a path to the RRs in cluster 1 if it happens that the best
overall route is learned from cluster 1. It doesn't however allow
other clusters to be aware of more than one path per prefix learned
by cluster 2.
Uttaro, et al. Expires Oct 25, 2016 [Page 16]
Internet-Draft draft-ietf-idr-add-paths-guidelines-08 Apr 2016
========== ==================
= = = =
= +---+ +---+ +---+ =
= |RR |---------------|RR | <-BE| | =
= |a | |y |------|PE1| =
= | | | | | | =
= +---+ +---+ +---+ =
= | = = | \ / =
= | = = | \ / =
= | = = | \/ =
= | = = | /\ =
= | = = | / \ =
= | = = | / \ =
= +---+ +---+ +---+ =
= |RR |---------------|RR |------| | =
= |b | |z | <-BE|PE2| =
= | | | | | | =
= +---+ +---+ +---+ =
= = = =
========== ==================
RR Cluster 1 RR Cluster 2
Figure 3: RR Cluster Before Add-Paths
The following sequence of steps occurs in the example migration
strategy:
1. The route reflectors are upgraded in each cluster, one by one, to
support Add-Paths. This allows the intra- and (eventually) inter-
cluster RR-to-RR sessions to start using Add-Paths. All RRs are
configured to use the Add-N, N=2 path selection algorithm. The
effect of this step is to slightly reduce convergence time when
the best and second-best paths for a prefix are learned by a
single cluster (such as cluster 2 in Figure 3).
2. The clients are upgraded in each cluster, one by one, to support
Add-Paths. On the RRs Add-Paths is configured to use the Add-N,
N=2 path selection algorithm towards upgraded client peers. At
this step clients are configured in the receive-only Add-Paths
mode. This means that best-external continues to operate as
before in the client-to-RR direction. The effect of this step is
to ensure that all clients have two paths per prefix for ECMP or
fast failover, assuming at least 2 paths are available.
3. The clients are re-configured to use Add-Paths in the transmit
direction towards their RR peers. This causes Add-Paths to replace
Uttaro, et al. Expires Oct 25, 2016 [Page 17]
Internet-Draft draft-ietf-idr-add-paths-guidelines-08 Apr 2016
the best-external behavior. The effect of this step is to free up
CPU and memory resources related to the storage of paths that are
third best or worse. If a cluster such as the one in Figure 3 had
50 clients, and 10 of these learned an external route for the same
prefix, then the RRs in that cluster would need to store up to 12
paths for that prefix. This would be true even if the 2 best
overall paths came from another cluster. Contrast this with the
use of Add-Paths in the client-to-RR direction. For the same case
the route reflectors need only store the 2 paths learned from non-
client peers.
5.2. Scalability Considerations
In terms of scalability, we note that advertising multiple paths per
prefix requires more memory and state than the current behavior of
advertising the best path only. A BGP speaker that does not implement
Add-Paths maintains send state information in its prefix data
structure per neighbor as a way to determine that the prefix has been
advertised to the neighbor. With Add-Paths, this information has to
be replicated on a per path basis that needs to be advertised.
Mathematically, if K is the number of neighbors, s is the "send
state" size per prefix in bytes, and N is the number of advertised
paths per prefix, then the current memory requirement for BGP "send
state" = K * s bytes; with Add-Paths, it becomes K * s * N bytes. In
practice, this value may be reduced with implementation optimizations
similar to attribute sharing. Receiving multiple paths per prefix
also requires more memory and state since each path is a separate
entry in the Adj-RIB-Ins.
5.3. Routing Consistency Considerations
As discussed in previous sections Add-Paths can help routers select
more optimal paths and it can help deal with certain route
oscillation conditions arising from incomplete knowledge of the
available paths. But depending on the path selection algorithm and
how it is used Add-Paths is not immune to its own cases of routing
inconsistencies. If the BGP routers within an AS do not make
consistent routing decisions about how to reach a particular
destination, route oscillations may occur and these route
oscillations may result in traffic loss.
Optimizing an Add-Paths deployment for scalability may run counter to
routing consistency goals, and in these circumstances operators have
to decide the correct tradeoff for their particular deployment. For
example the Advertise All Paths mode, if applied to many prefixes, is
far from ideal from a scalability perspective but it does guarantee
routing consistency and correctness. A path selection mode that
Uttaro, et al. Expires Oct 25, 2016 [Page 18]
Internet-Draft draft-ietf-idr-add-paths-guidelines-08 Apr 2016
allows better control over scalability is the Advertise N paths mode,
but this is susceptible to routing inconsistency. First, if the N
paths do not include the best path from each neighbor AS group then
route oscillation cannot be precluded. Second, if the advertising
router (e.g. an RR) advertises N paths to peer_n and M paths to
peer_m, and N < M, care must be exercised to ensure that all paths
advertised to peer_n are included in the paths advertised to peer_m.
This can be assured as long as the advertising router has strictly
ordered all of its paths.
5.4. Consistency between Advertised Paths and Forwarding Paths
When using Add-Paths, routers may advertise paths that they have not
selected as best, and that they are thus not using for traffic
forwarding. This is generally not an issue if encapsulation is used
in the AS as described in [RFC4364] and all forwarding decisions,
including by the tunnel egress router, are based on label information
- i.e. if only the ingress router performs an IP FIB lookup. In this
situation the dataplane path followed by the packets is the one
intended by the ingress router, and corresponds to the control plane
path it selected.
On the other hand, if Add-Paths is used in a network without
encapsulation, some scenarios can result in forwarding deflection or
loops. Such forwarding anomalies already occur without Add-Paths,
when the routers on the forwarding path do not have a synchronized
view of the best path. They will deflect the traffic to their own
local view of the best path, and, when multiple deflections occur,
forwarding loops can occur. With Add-Paths, the issue can be
exacerbated due to routers advertising non-best paths. As discussed
above, encapsulation can help with this issue, but only to the extent
that it allows downstream routers to forward without an IP FIB
lookup.
A first example of such issue is when the Local-Pref of non-primary
paths received over IBGP sessions is modified. The ingress router
may thus select as best a path non-preferred by the egress, and the
egress router will thus deflect the traffic.
Another example is when the best path is selected based on tie-
breaking rule. When the ingress and the egress base their path
selection on the router-id of the neighbor that advertised the path
to them, the result may be different for each of them. This specific
issue is described and solved in [draft-pmohapat].
In general, if the network forwards on a hop-by-hop basis and does
not make use of encapsulation, it is necessary to advertise the best
Uttaro, et al. Expires Oct 25, 2016 [Page 19]
Internet-Draft draft-ietf-idr-add-paths-guidelines-08 Apr 2016
path. The second path that is advertised should be the second best
path using one of the path selection modes described previously.
Additional paths are discretionary with the presumption that they can
be forwarded on a hop-by-hop basis.
Similarly, if the network uses encapsulation, the best path should be
advertised for consistency, the second best path should be advertised
for fast routing convergence. All further paths and their choice for
selection are completely discretionary; the destination is presumed
to be reachable via encapsulation.
5.5. Interactions with Route Filtering
As noted in the previous section, modification of advertised paths
may lead to inconsistent route selection. This is true even when the
Add-Paths feature is not in use. Similarly, the use of route
filtering, when used carelessly for IBGP, may result in inconsistent
route selection in an AS with the possibility of introducing
forwarding loops.
The Add-Paths feature has additional considerations for route
filtering since the receiver of multiple paths is unable to determine
by inspection of the received NLRI which path corresponds to the
sender's active path for the prefix. The sender SHOULD send the best
path when sending multiple paths for a destination. The receiver must
take care when rejecting destinations to not discard the best path
but permit alternate paths. A failure on either the part of the
sender or receiver to distribute/receive the best path may result in
inconsistent route selection.
An implementation MAY support the ability to suppress advertisement
of all alternate paths when the export policy would otherwise
suppress the best path.
5.6. Routing Churn
As noted in section 3.3 using Add-Paths between IBGP peers can help
to reduce routing churn with EBGP peers. This benefit does however
come at the cost of potentially increased churn between the IBGP Add-
Paths peers. In a non Add-Paths deployment a change in the preference
order of non-best paths requires no updates to be sent to peers. But
when a router has Add-Paths peers changes in non-best path preference
may no longer be invisible and increased route churn may be
observable. Choosing the right path selection mode and parameters -
for example not setting N unnecessarily large in the Add-N mode, is
important to minimizing this additional churn.
Uttaro, et al. Expires Oct 25, 2016 [Page 20]
Internet-Draft draft-ietf-idr-add-paths-guidelines-08 Apr 2016
6. Security Considerations
This document introduces no new security concerns in the base
operation of BGP [RFC4271].
7. Acknowledgments
This document was prepared using 2-Word-v2.0.template.dot.
8. Contributors
The following individuals are acknowledged for their contributions to
earlier versions of this draft: Pradosh Mohapatra, Virginie Van den
Schrieck and Rohit Gupta.
9. IANA Considerations
IANA has assigned capability number 69 for the ADD-PATH Capability
described in this document. This registration is in the BGP
Capability Codes registry.
10. References
10.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, DOI
10.17487/RFC2119, March 1997, <http://www.rfc-
editor.org/info/rfc2119>.
[RFC4271] Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
Border Gateway Protocol 4 (BGP-4)", RFC 4271, DOI
10.17487/RFC4271, January 2006, <http://www.rfc-
editor.org/info/rfc4271>.
[RFC4364] Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual
Private Networks (VPNs)", RFC 4364, DOI
10.17487/RFC4364, February 2006, <http://www.rfc-
editor.org/info/rfc4364>.
Uttaro, et al. Expires Oct 25, 2016 [Page 21]
Internet-Draft draft-ietf-idr-add-paths-guidelines-08 Apr 2016
10.2. Informative References
[Add-Paths] Walton, D., Retana, A., Chen E., Scudder J.,
"Advertisement of Multiple Paths in BGP", draft-
ietf-idr-add-paths-13, Dec 11, 2015.
[draft-pmohapat] Mohapatra, P., Fernando, R., Filsfils, C., and R.
Raszuk, "Fast Connectivity Restoration Using BGP
Add-path", draft-pmohapat-idr-fast-conn-restore-
02.txt, Oct 3, 2011.
[BEST-EXT] Marques, P., Fernando, R., Chen, E., Mohapatra, P.,
Gredler, H., "Advertisement of the best external
route in BGP", draft-ietf-idr-best-external-05.txt,
Jan 3, 2012.
[oscillation] Walton, D., Retana, A., Chen, E., Scudder, J., "BGP
Persistent Route Oscillation Solutions", draft-
walton-bgp-route-oscillation-stop-06.txt, June 14,
2012.
[BGP-ORR] Raszuk, R., Cassar, C., Aman, E., Decraene, B.,
Litkowski, S., Wang, K., "BGP Optimal Route
Reflection (BGP-ORR)", draft-ietf-idr-bgp-optimal-
route-reflection-11, Jan 8, 2016.
Uttaro, et al. Expires Oct 25, 2016 [Page 22]
Internet-Draft draft-ietf-idr-add-paths-guidelines-08 Apr 2016
Appendix A. Other Path Selection Modes
A.1. Advertise Neighbor-AS Group Best Path
[walton-osc] proposes that a router groups its paths based on the
neighbor AS from which it was learned, and to advertise the best path
in each of those groups.
The control plane stress induced by this solution is the computation
of the per-neighbor path group, and the application of the decision
process to each of them. The Control-Plane load is bounded by the
number of neighboring ASes advertising a prefix, which cannot be
known a-priori.
Path optimality and backup path optimality are not guaranteed, as the
paths advertised are not all the AS-wide preferred paths. Backup path
availability is not guaranteed. Indeed, if only one AS advertises
this prefix, even on multiple EBGP sessions, only one of the paths
may be selected and advertised.
A.2. Best LocPref/Second LocPref
This selection method consists in grouping the paths by Local
Preference. A router sends to its peers all paths with the highest
Local Preference. If there is only a single path with the highest
Local Preference, it also sends all paths with the second best Local
Preference.
This method ensures that all routers know all paths with the best
local preference. As local preference are often related to the type
of peering of the peer the path comes from, this ensures that in case
of failure, routers have a backup path of equivalent quality. This
prevents for example that a router switches temporarily on a peer
path while an alternate path from a customer is available but hidden
at the border of the AS. Such a situation could result in a
temporary withdrawal of the prefix on some EBGP sessions when the
router selects the path via the peer.
The advertisement of the Second Local Preference occurs when there is
no alternate path with the same quality as the best path. This way,
fast convergence is still ensured. Backup path is optimal, as it has
the second AS-Wide preference, which becomes the AS-wide best
preference upon failure of the primary one.
Sending all the paths with a given Local Preference also has a
positive impact on routing optimality. Indeed, this allows border
Uttaro, et al. Expires Oct 25, 2016 [Page 23]
Internet-Draft draft-ietf-idr-add-paths-guidelines-08 Apr 2016
routers to have an increased path visibility and to choose their best
path based on their own criteria.
The computational cost of this solution is reduced when there are
several paths with the best local preference. In this case, it is
sufficient to stop the decision process after the first rule to have
the set of paths to be advertised. When it is necessary to advertise
the paths with second local-preference, the additional cost is to
apply a second time the first rule of the decision process, which is
still reasonable. The memory cost depends on the number of paths
with the best local preference.
A.3. Advertise Paths at decisive step -1
When the goal is to provide fast recovery by advertising candidate
post-reconvergence paths, one can choose to stop the decision process
just before the step where only one path remains. If the decision
process comes to IGP tie-break, all remaining paths are advertised.
This way, routers advertise as many paths as possible with a quality
as similar as possible.
This path selection is an intermediary solution between the two
preceding ones. Here, instead of stopping the decision process at
the local preference step or the IGP step, we stop it before the rule
that removes the best potential backup paths. This way, we minimize
the number of paths to advertise while guaranteeing the presence of a
backup path. Primary and backup path optimality is ensured, as all
paths with the same AS-wide preference as the best paths are included
in the set of paths advertised.
Uttaro, et al. Expires Oct 25, 2016 [Page 24]
Internet-Draft draft-ietf-idr-add-paths-guidelines-08 Apr 2016
Authors' Addresses
Jim Uttaro
AT&T
200 S. Laurel Avenue
Middletown, NJ 07748 USA
Email: uttaro@att.com
Pierre Francois
Institute IMDEA Networks
Avda. del Mar Mediterraneo, 22
Leganese 28918
ES
Email: pierre.francois@imdea.org
Pradosh Mohapatra
Cumulus Networks
pmohapat@cumulusnetworks.com
Roberto Fragassi
Alcatel-Lucent
600 Mountain Avenue
Murray Hill, New Jersey
Email: roberto.fragassi@alcatel-lucent.com
Adam Simpson
Alcatel-Lucent
600 March Road
Ottawa, Ontario K2K 2E6
Canada
Email: adam.simpson@alcatel-lucent.com
Keyur Patel
Cisco Systems
170 W. Tasman Drive
San Jose, CA 95134 USA
Email: keyupate@cisco.com
Jeffrey Haas
Juniper Networks
1194 N. Mathilda Ave.
Sunnyvale, CA 94089
USA
Email: jhaas@juniper.net
Uttaro, et al. Expires Oct 25, 2016 [Page 25]
